Integrated NGL recovery and liquefied natural gas production

ABSTRACT

The separation of methane from an admixture ( 110 ) with ethane and higher hydrocarbons, especially natural gas, using a scrub column ( 114 ), in which the admixture is separated into a methane-rich overhead ( 116 ) that is partially condensed ( 122 ) to provide reflux to the column ( 114 ) and liquid methane-depleted bottoms liquid ( 126 ), is improved by providing additional reflux ( 136 ) derived from an ethane enriched stream ( 130 ) from fractionation ( 128 ) of the bottoms liquid. Preferably, absorber liquid ( 140 ) from the fractionation ( 128 ) also is introduced into the scrub column. The vapor fraction ( 120 ) remaining after partial condensation can be liquefied ( 122 ) to provide LNG product ( 124 ).

BACKGROUND OF THE INVENTION

The present invention relates to the separation of methane fromadmixture with ethane and heavier hydrocarbons and has particular, butnot exclusive, application to an integrated process in which natural gasliquids (NGL) are recovered and liquefied natural gas (LNG) producedfrom natural gas (NG).

Natural gas comprises primarily methane and minor constituents thatinclude heavier hydrocarbons. Liquefied natural gas contains mostlymethane. The hydrocarbons heavier than methane are usually condensed andrecovered as natural gas liquids and fractionated to yield valuablehydrocarbon products.

A typical NG liquefaction system comprises a scrub column fed with rawnatural gas or pipeline gas and producing a methane-rich overhead vaporand NGL as bottoms liquid. A portion of methane-rich overhead vapor ispartially condensed to provide reflux for the column and the remainderliquefied to provide the LNG product. The bottoms liquid is fractionatedto obtain individual hydrocarbons and/or hydrocarbon cuts (fractions) asvaluable products.

Efficiency of liquefaction improves with increasing pressure andaccordingly the NG liquefaction pressure should be significantly abovethe critical pressure of methane in order to minimize power consumptionof the LNG process. However, recovery of heavy hydrocarbons by the scrubcolumn becomes more difficult with increasing pressure and it is notpossible to separate a mixture at a pressure above its criticalpressure. Hence, the scrub column has to operate significantly below thecritical pressure of methane in order to achieve satisfactoryseparation. A common solution is to expand the scrub column feed andthen to compress the overhead vapor. Work obtained from the isentropicexpansion of the feed can be used to at least partially drive theoverhead compressor(s). Such solution is shown in U.S. Pat. No.4,065,278 (published Dec. 27, 1977).

Expansion of the scrub column feed followed by compression of theoverhead vapor can be avoided by recycling heavy components obtainedfrom NGL fractionation to the top or near the top of the scrub column asabsorber liquid. For example, Chen-Hwa Chiu (Oil and Gas Journal, Nov.24, 1997, 56-63) reports that the use of a heavy alkane recycle, such asall or part of a C₄ NGL fraction, to the scrub column of a LNG processcan raise the critical pressure of the separated mixture and thus theoperating pressure for the scrub column. In an exemplified process,there is partial or total recycle of C₄ NGL fraction recovered from adebutanizer.

WO 01101307/US-A 2003005722/U.S. Pat. No. 6,742,358 (published Dec. 2,2002/Jan. 9, 2003/Jun. 1, 2004) discloses LNG processes in which topreflux to the scrub column is provided by condensing vapor withdrawnfrom an intermediate location of the column. It also discloses processesin which the vapor and liquid fractions of partially condensed feed gasare separately fractionated and bottoms liquid from fractionation of thevapor fraction provides intermediate or top reflux to the fractionationof the liquid fraction. In all of these processes, vapor overhead fromthe scrub column is compressed before liquefaction.

DE-A-10205366 (published Aug. 21, 2003) discloses an LNG process inwhich ethane-rich vapor overhead from the scrub column is cooled andpassed to a second column for the removal of residual higherhydrocarbons. The bottoms liquid of the second column provides reflux tothe scrub column. Preferably, a C₄/C₅ NGL fraction provides reflux tothe second column. The rectification and adsorption functions of the twocolumns can be combined into a single column.

U.S. Pat. No. 6,662,589/EP-A-1469266 (published Dec. 16, 2003/Oct. 20,2004) discloses an LNG process in which a NGL fraction comprisingcomponents heavier than ethane is fed to the scrub column as absorberliquid at a location between the natural gas feed and feed of amethane-rich reflux stream. In the exemplified embodiment, the refluxstream is obtained by partial condensation of the overhead vapor fromthe scrub column. The scrub column overhead is not compressed prior toliquefaction to provide the LNG product.

WO 2004/010064 (published Jan. 29, 2004) discloses an LNG process inwhich a C₄/C₅ NGL fraction is directly or indirectly fed to the scrubcolumn to provide additional reflux. The fraction is fed to the columnat or above the feed of reflux provided by partial condensation of thecolumn vapor overhead.

Critical pressure of the mixture in the scrub column can be increasedalso by refluxing the column with an ethane-rich stream. This alsoallows good C₂-C₃ separation and high propane (C₃) recovery from NGL.

WO-A-0188447/U.S. Pat. No. 6,401,486 (published Nov. 22, 2001/Jun. 11,2002) discloses an LNG process in which top reflux for the scrub columnis provided by condensation of a vapor containing mostly methane andethane with very little propane. The scrub column overhead vapor istotally liquefied to provide the LNG product and the scrub columnbottoms is fractionated in an NGL purifying column. The vapor condensedto provide said top reflux can be derived from:

-   -   (i) vapor overhead from the NGL fractionation and optionally        flashed vapor obtained by flashing the liquefied, and preferably        sub-cooled, scrub column overhead to near atmospheric pressure;    -   (ii) a slipstream of the feed gas portion;    -   (iii) flashed vapor obtained by flashing the liquefied, and        preferably sub-cooled, scrub column overhead to near atmospheric        pressure; or    -   (iv) a portion of the liquefied, and preferably sub-cooled,        scrub column overhead.        In options (i), (iii) and (iv), additional reflux to the scrub        column can be provided by condensing a slipstream of the feed        gas portion but there is no teaching of providing reflux derived        from both NGL fractionation and partially condensed scrub column        overhead. In these processes, it is unnecessary to compress the        scrub column overhead prior to liquefaction.

EP-A-0178207/U.S. Pat. No. 4,690,702 (published Apr. 15, 1986/Sep. 1,1987); DE-A-3802553/U.S. Pat. No. 4,952,305 (published Aug. 3, 1989/Aug.28, 1990); and EP-A-0535752/U.S. Pat. No. 5,291,736 (published Apr. 7,1993/Mar. 8, 1994) all disclose LNG processes in which reflux to thescrub column is provided by a mixture of methane and ethane obtained bycondensing vapor overhead from an NGL fractionation. None of thesepatents show reflux obtained by partially condensing the overheadproduct.

It has now been found that the NGL and natural gas product recovery canbe improved by combining the benefits of the reflux obtained bypartially condensing the scrub column overhead with the benefits ofethane-rich reflux and absorber liquid in an efficient manner, from thestandpoint of thermodynamic efficiency, simplicity of equipment, and therecovery of valuable components such as propane and butane.

BRIEF SUMMARY OF THE INVENTION

In its broadest aspect, the present invention provides a process forrecovery of components heavier than methane from a feed of methane inadmixture with ethane and heavier hydrocarbon(s), the processcomprising:

-   -   introducing the feed into a scrub column at a first location;    -   withdrawing from the scrub column a first overhead vapor stream        depleted in components heavier than methane and a bottoms stream        enriched in components heavier than methane;    -   cooling and partially condensing the first overhead vapor stream        to form a first two-phase stream;    -   separating the first two-phase stream to provide a second        overhead vapor stream and a methane-rich first reflux stream;    -   introducing the methane-rich first reflux stream at a second        location in the scrub column above the first location;    -   separating the bottoms stream into an ethane-enriched stream and        one or more streams enriched in components heavier than ethane;        and    -   introducing into the scrub column, at a location selected from        the second location and a third location above the first        location, an ethane-enriched second reflux stream derived from        the ethane-enriched stream.

In a preferred embodiment of this invention, the ethane-enriched stream,preferably deethanizer overhead, obtained from NGL fractionation iscondensed, pumped, combined with the reflux stream obtained by partiallycondensing the scrub column overhead vapor and recycled to the scrubcolumn, preferably to a scrub column reflux drum. This allows the scrubcolumn to operate at a higher pressure by increasing the mixture'scritical pressure and also improves ethane-propane separation. Theethane-enriched stream can be fully condensed using mixed refrigerant(MR) refrigeration available in the main heat exchanger of the LNGprocess to maximize the benefit.

Use of a heavy recycle stream, particularly pentane and isopentane, canalso be beneficial. Such a stream can be introduced either to the refluxdrum or directly to the scrub column. The heavy and light recycles canbe combined and cooled separately or, preferably, mixed with the refluxstream obtained by condensing the scrub column overhead vapor. Refluxobtained by condensing the overhead vapor is typically more than about80% of the total liquid reflux (including any heavy recycle stream) tothe scrub column. In the preferred implementation cooling takes place inthe warm bundle of the main LNG process heat exchanger.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

In the drawings:

FIG. 1 shows one embodiment of the present invention;

FIG. 2 shows a modification of the embodiment of FIG. 1, in which thereflux drum (118) is replaced by an absorption column (218);

FIG. 3 shows another modification of the embodiment of FIG. 1, in whichthe ethane-enriched stream (130) and “absorber liquid” stream (140) arecombined to form a single stream (330);

FIG. 4 shows a modification of the embodiment of FIG. 3, in which thecombined second reflux and absorber liquid stream (330) is phaseseparated (430);

FIG. 5 shows a modification of the embodiment of FIG. 4, in which theseparated vapor portion (436) is compressed, cooled and condensed andthe resultant stream (536) combined with the liquid portion (438); and

FIG. 6 shows another modification of the embodiment of FIG. 4, in whichthe overhead vapor (116) from the scrub column (114) is condensed in twostages (612, 122) to provide separate reflux streams (619, 626) to thescrub column.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention provides, in its broadestprocess aspect, a process for recovery of components heavier thanmethane from a feed of methane in admixture with ethane and heavierhydrocarbon(s), the comprising:

-   -   introducing the feed into a scrub column at a first location;    -   withdrawing from the scrub column a first overhead vapor stream        depleted in components heavier than methane and a bottoms stream        enriched in components heavier than methane;    -   cooling and partially condensing the first overhead vapor stream        to form a first two-phase stream;    -   separating the first two-phase stream to provide a second        overhead vapor stream and a methane-rich first reflux stream;    -   introducing the methane-rich first reflux stream at a second        location in the scrub column above the first location;    -   separating the bottoms stream into an ethane-enriched stream and        one or more streams enriched in components heavier than ethane;        and    -   introducing into the scrub column, at a location selected from        the second location and a third location above the first        location, an ethane-enriched second reflux stream derived from        the ethane-enriched stream.

In a corresponding apparatus aspect, the present invention provides anapparatus for recovery of components heavier than methane from a feed ofmethane in admixture with ethane and heavier hydrocarbon(s) by a processof the aforementioned aspect, said apparatus comprising:

-   -   a scrub column;    -   conduit means for introducing the feed into the scrub column at        a first location;    -   conduit means for withdrawing from the scrub column a first        overhead vapor stream depleted in components heavier than        methane and a bottoms stream enriched in components heavier than        methane;    -   heat exchanger means for cooling and partially condensing the        first overhead vapor stream to form a first two-phase stream;    -   separation means for separating the first two-phase stream to        provide a second overhead vapor stream and a methane-rich first        reflux stream;    -   conduit means for introducing the methane-rich first reflux        stream at a second location in the scrub column above the first        location;    -   separation means for separating the bottoms stream into an        ethane-enriched stream and one or more streams enriched in        components heavier than ethane; and    -   conduit means for introducing into the scrub column, at a        location selected from the second location and a third location        above the first location, an ethane-enriched second reflux        stream derived from the ethane-enriched stream.

In a preferred process aspect, the invention provides a process forobtaining liquefied methane from a feed of methane in admixture withethane and heavier hydrocarbon(s), said process comprising:

-   -   introducing the feed into a scrub column at a first location;    -   withdrawing from the scrub column a first overhead vapor stream        depleted in components heavier than methane and a bottoms stream        enriched in components heavier than methane;    -   cooling and partially condensing the first overhead vapor stream        to form a first two-phase stream;    -   separating the first two-phase stream to provide a second        overhead vapor stream and a methane-rich first reflux stream;    -   liquefying the second overhead vapor stream;    -   introducing the methane-rich first reflux stream at a second        location in the scrub column above the first location;    -   separating the bottoms stream into an ethane-enriched stream and        one or more streams enriched in components heavier than ethane;        and    -   introducing into the scrub column, at a location selected from        the second location and a third location above the first        location, an ethane-enriched second reflux stream derived from        the ethane-enriched stream.

In a preferred apparatus aspect, the present invention provides anapparatus for obtaining liquefied methane from a feed of methane inadmixture with ethane and heavier hydrocarbon(s) by a process accordingto said preferred process aspect, said apparatus comprising:

-   -   a scrub column;    -   conduit means for introducing the feed into the scrub column at        a first location;    -   conduit means for withdrawing from the scrub column a first        overhead vapor stream depleted in components heavier than        methane and a bottoms stream enriched in components heavier than        methane;    -   heat exchanger means for cooling and partially condensing the        first overhead vapor stream to form a first two-phase stream;    -   separation means for separating the first two-phase stream to        provide a second overhead vapor stream and a methane-rich first        reflux stream;    -   heat exchange means for liquefying said second overhead vapor        stream;    -   conduit means for introducing the methane-rich first reflux        stream at a second location in the scrub column above the first        location;    -   separation means for separating the bottoms stream into an        ethane-enriched stream and one or more streams enriched in        components heavier than ethane; and    -   conduit means for introducing into the scrub column, at a        location selected from the second location and a third location        above the first location, an ethane-enriched second reflux        stream derived from the ethane-enriched stream.

As indicated above, it is preferred that the feed is a cooled naturalgas feed and the second overhead vapor is liquefied to provide aliquefied natural gas product.

The ethane-enriched second reflux stream can be fed to the scrub columnseparately from the methane-rich first reflux stream but it is preferredthat it is mixed with the methane-rich first reflux stream beforeintroducing into the scrub column. The ethane-enriched stream can bepartially or fully condensed before mixing with the methane-rich firstreflux stream. The mixing suitably takes place upstream of or in areflux drum or by feeding the first two-phase stream to the bottom of anabsorption column to which the ethane-enriched second reflux stream isfed as reflux.

The ethane-enriched stream, alone or after admixture with one or moreother process streams preferably is condensed at temperature below thatof the feed to the scrub column and the condensed stream pumped prior tointroduction into the scrub column as the ethane-enriched second refluxstream. Said temperature usually is below −32° F. (−35.5° C.).

Usually, the ethane-enriched stream (130) is the vapor overhead of adeethanizer. Methane can be removed from the ethane-enriched streamwhereby the second reflux stream consists essentially of ethane.Preferably, the second reflux stream contains less than about 0.05% ofpropane.

Usually, the methane-rich first reflux stream will constitute at leastabout 80% of the total reflux (i.e. liquid fed to the scrub column abovethe first location) and the second reflux stream will be less than about20% of the total reflux.

Preferably more than 90%, especially more than 96%, of propane and/orbutane contained in the feed is recovered from the bottoms stream asproduct.

As illustrated in FIG. 6, the first overhead stream can be partiallycondensed in two stages and liquid fraction from each condensation fedto the scrub column as reflux.

In a preferred embodiment, a process of the invention comprises

-   -   introducing the feed into a scrub column at a first location;    -   withdrawing from the scrub column a first overhead vapor stream        depleted in components heavier than methane and a bottoms stream        enriched in components heavier than methane;    -   cooling and partially condensing the first overhead vapor stream        to form a first two-phase stream;    -   separating the first two-phase stream to provide a second        overhead vapor stream and a methane-rich first reflux stream;    -   introducing the methane-rich first reflux stream at a second        location in the scrub column above the first location;    -   separating the bottoms stream into an ethane-enriched stream and        two or more streams enriched in components heavier than ethane        including an absorber liquid stream enriched in component(s)        heavier than ethane;    -   introducing into the scrub column, at a location selected from        the second location and a third location above the first        location, an ethane-enriched second reflux stream derived from        the ethane-enriched stream; and    -   introducing the absorber liquid into the scrub column, at a        location selected from the second location, the third location        and a fourth location above the first location.

In a corresponding preferred apparatus embodiment, an apparatus of theinvention comprises:

-   -   a scrub column;    -   conduit means for introducing the feed into the scrub column at        a first location;    -   conduit means for withdrawing from the scrub column a first        overhead vapor stream depleted in components heavier than        methane and a bottoms stream enriched in components heavier than        methane;    -   heat exchanger means for cooling and partially condensing the        first overhead vapor stream to form a first two-phase stream;    -   separation means for separating the first two-phase stream to        provide a second overhead vapor stream and a methane-rich first        reflux stream;    -   conduit means for introducing the methane-rich first reflux        stream at a second location in the scrub column above the first        location;    -   separation means for separating the bottoms stream into an        ethane-enriched stream and two or more streams enriched in        components heavier than ethane including an absorber liquid        stream enriched in component(s) heavier than ethane;    -   conduit means for introducing into the scrub column, at a        location selected from the second location and a third location        above the first location, an ethane-enriched second reflux        stream derived from the ethane-enriched stream; and    -   conduit means for introducing the absorber liquid into the scrub        column, at a location selected from the second location, the        third location and a fourth location above the first location.

All of the features discussed above in connection with the broadestaspects apply to this preferred embodiment.

The absorber liquid can comprise C₄ hydrocarbon(s) but preferablycomprises C₅₊ hydrocarbon(s)

The absorber liquid can be fed to the scrub column separately fromeither the ethane-enriched second reflux stream or the methane-richfirst reflux stream. However, it is preferred that it is combined withat least one of the methane-rich first reflux stream and theethane-enriched second reflux stream (136) before introducing into thescrub column. For example, it can be combined with at least one of thefirst two-phase stream and the ethane-enriched second reflux streamupstream of or in a reflux drum or the first two-phase stream can be fedto the bottom of an absorption column to which the absorber liquid andoptionally the ethane-enriched second reflux stream is fed as reflux.The absorber liquid can be combined with the first overhead vapor streambefore partial condensation of said stream to form the first two-phasestream and/or combined with gaseous ethane-enriched stream beforecondensation of said stream to provide the second reflux stream.

A combined absorber liquid and gaseous ethane-enriched stream can phaseseparated and the liquid fraction fed to the scrub column above thefirst location. The liquid fraction can be combined with the firstoverhead stream before partial condensation of that overhead stream toprovide the first two-phase stream. The vapor fraction can be condensedand the condensed stream fed to the scrub column above the firstlocation, added to the second overhead vapor prior to liquefaction, orcombined with the first overhead stream before partial condensation ofthat overhead stream to provide the first two-phase stream. Both thecondensed vapor fraction and the liquid fraction can be combined withthe first overhead stream before partial condensation of that overheadstream to provide the first two-phase stream.

Usually, the absorber liquid will constitute less than about 10% of thetotal reflux (i.e. liquid fed to the scrub column above the firstlocation).

The following is a description by way of example only and with referenceto the accompanying drawings of presently preferred embodiments of theinvention.

Referring to FIG. 1, pretreated pressurized natural gas feed 110containing primarily methane with heavier hydrocarbons in the C₂-C₆range with very little water, acid gases such as CO₂ and H₂S, and othercontaminants such as mercury is cooled in a heat exchanger 112 tobetween about −20° F. (−29° C.) and about −40° F. (−40° C.) and fed tothe scrub column 114. Typically the feed 110 is at a pressure of betweenabout 600 and about 900 psia (4 and 6.25 MPa) and at about ambienttemperature. Heat exchanger 112 represents multiple stages of cooling byevaporating propane at different pressures. Any other means of cooling,such as vaporizing mixed refrigerant in a single exchanger, can be used.Stream 110, or a vapor portion of stream 110 downstream of the heatexchanger, 112 can be throttled or isentropically expanded into thecolumn 114. Energy obtained from the expansion can be used to at leastpartially compress another vapor stream, for example process stream 116,120, 150, or 156.

Scrub column 114 separates the feed into a bottoms liquid 126 & 127enriched in heavier hydrocarbons and a “first” overhead vapor stream 116enriched in methane. One portion 127 of the bottoms liquid is vaporizedin reboiler 128 to provide boilup for the column 114. The reboiler 128can use a portion of feed stream 110, or any other suitable processstream, to provide heat duty. The column may also have an intermediatereboiler, for which the feed stream portion also can provide heat duty.The remaining bottoms liquid 126, generally described as Natural GasLiquid (NGL), is fed to NGL fractionation system 128. There, NGL isusually reduced in pressure and separated using known separationapparatus such as deethanizer, depropanizer, and/or debutanizer toprovide two or more hydrocarbon fractions. The bottoms liquid 126 isseparated into a stream (the ethane-enriched stream) containing methaneand ethane with very little propane and factions containing primarilyC₃, C₄, and C₅₊ hydrocarbons (i.e. n-pentane, isopentane and heavier).Typically, the ethane-enriched stream 130 is deethanizer overhead andcontains less than about 0.05% propane.

The use of the ethane-enriched second reflux stream 136 allows highrecovery of propane (96-99%) and butane (almost 100%) in thefractionation system.

A portion of C₅₊ hydrocarbons is withdrawn as “absorber” liquid 140,which is pumped to scrub column pressure (i.e. pressure sufficient tointroduce it to the scrub column 114, including equipment pressure dropsand static pressure) by pump 142, cooled in heat exchanger 144 againstvaporizing propane, further cooled in the main heat exchanger 122, andintroduced into a reflux drum 118, either mixed with the second refluxstream obtained from NGL fractionation or, as shown in ghost lines,directly. Heat exchanger 144 can be placed before or after the pump 142.

Mixing the absorber liquid 140 with the second reflux stream 136 priorto introduction into the reflux drum 118 is a preferred implementationas it allows equilibration and some absorption to take place in theconduit.

As shown in ghost lines, the absorber liquid 140 could be fed directlyto the top or near the top of the scrub column 114, or, in a preferredimplementation, combined with the first overhead vapor stream 116upstream of the main heat exchanger 122.

Ethane-enriched stream 130 is cooled and partially condensed in the heatexchanger 132 against vaporizing propane, cooled and completelycondensed in the main heat exchanger 122, pumped to the scrub columnpressure by pump 134, preferably combined with the absorber liquid 140,and introduced into the reflux drum as stream 136. Any uncondensed vaporupstream of the pump 134 can be separated, condensed in the middlebundle of the main heat exchanger 122, and combined with the liquefiednatural gas product 124.

Absorber liquid 140 can also be obtained from lighter products of NGLfractionation such as C₃ and C₄ hydrocarbons, either pure or blendedtogether. It may contain mostly C₅ hydrocarbons without C₆ and heaviercomponents that may be rejected in an additional distillation column.

Stream 130 can be nearly-pure ethane, methane being rejected in anadditional distillation column. A portion of ethane or ethane-methanemixture can be recovered as product.

The first overhead vapor stream 116 is cooled and partially condensed inthe warm bundle of the main heat exchanger 122 and introduced to thereflux drum 118. It can be compressed (not shown) prior to cooling inmain heat exchanger 122. The liquid portion is returned to the scrubcolumn as the “first” liquid reflux 119. The methane-enriched “second”vapor portion 120 is liquefied and preferably subcooled in the middleand cold bundle of the main heat exchanger to provide LNG product 124.

In a preferred implementation, the partially condensed first overheadvapor stream 116 is combined with the second reflux stream 136 and/orthe absorber liquid 140 in or upstream of the reflux drum 118 so thatsome equilibration takes place. Thus, the first liquid reflux (theliquid portion of the partially condensed first overhead vapor) getsmixed with the second liquid reflux 136 and/or the absorber liquid 140.

Typically, depending on natural gas feed composition, the second refluxstream 136 is less than about 20% of the total reflux (inclusive of anyabsorber liquid), and the absorber liquid 140 is less than about 10% ofthe total reflux. If the natural gas feed 110 does not containcomponents that are suitable for the absorber liquid 140 or does notcontain them in sufficient quantity they can be introduced as anadditional feed.

The second vapor stream 120 may be compressed (not shown) prior tointroducing into main heat exchanger 122 and/or reduced in pressurebefore subcooling. If the LNG product 124 is stored at high pressure(PNGL) there is no need for subcooling in the cold bundle.

The main heat exchanger 122 is cooled by vaporizing a recycled mixedrefrigerant (MR) stream 150, which is compressed, cooled by multiplestages of vaporizing propane, and separated into a liquid 152 and alighter vapor 156 (compression, cooling, and phase separation notshown). Vapor 156 is condensed, cooled, and expanded through throttlingvalve 158. Liquid 152 is cooled, expanded through throttle valve 154,and combined with vaporizing condensed vapor 156. The combined MRstreams are completely vaporized and leave the main heat exchanger 122as stream 150. Throttling valves 154 and/or 156 can be replaced withisentropic dense fluid expanders, such as hydraulic turbines. Any otherrefrigeration system or a combination of systems, including pure fluidcascade and isentropic vapor expansion as described in U.S. Pat. No.6,308,531, can be used to refrigerate the main heat exchanger 122.

FIG. 2 shows a modification of the embodiment of FIG. 1, where thereflux drum 118 is replaced with an absorption column 218. Absorberliquid 140 and/or the second reflux stream 136, preferably both combinedinto stream 136, are fed to the top of the absorption column 218. Theymay also enter the column independently at the same location or atdifferent locations with at least one of the two streams fed to the topof the absorption column 218. For example, absorber liquid 140 can befed some stages below the top of the column or at the bottom of thecolumn. The second overhead vapor stream 120 is withdrawn from the topof the column 218 and the first reflux stream 119 is withdrawn from thebottom of the column. Multiple stages in the column 218 improve theabsorption of heavy components from the ascending vapor.

FIG. 3 shows another modification of the embodiment of FIG. 1, in whichthe ethane-enriched stream 130 and absorber liquid 140 are combined toform a single stream 330. Stream 330 is cooled and partially condensedin heat exchanger 332 against vaporizing propane, further cooled andcompletely condensed in the main heat exchanger 122, pumped to the scrubcolumn pressure in pump 334, and introduced into the reflux drum. Themixing of streams 130 and 140 at a warmer temperature and condensingthem together is thermodynamically more efficient than theconfigurations shown in FIGS. 1 and 2. The benefit is similar to thebenefit of the absorption column 218 as absorption takes place in heatexchangers 332 and 122. This configuration also eliminates passages inthe main heat exchanger 122. As with the configuration of FIG. 1,streams 116 and 330 can be combined downstream of the main heatexchanger 122 and before the reflux drum 118.

FIG. 4 shows a modification of the embodiment of FIG. 3, in which thecombined ethane-enriched stream and absorber liquid stream 330 is fed toa phase separator 430. The liquid portion 438 is pumped by pump 432 tothe pressure of the scrub column 114 and combined with the firstoverhead vapor 116 upstream of the main heat exchanger 122. The combinedstream 416 exiting the main heat exchanger 122 is then fed to the refluxdrum 118. The smaller vapor portion 436 is condensed in the main heatexchanger 122 and either pumped by pump 434 and introduced to the refluxdrum 118, optionally combined with stream 416, or combined withliquefied natural gas upstream of the subcooling portion (cold bundle)of the main heat exchanger 122, where the liquid can be reduced inpressure prior to subcooling. Combining both streams 130 & 116 and theabsorber liquid 140 upstream of the main heat exchanger 122 furtherincreases thermodynamic efficiency of the process.

As an option, stream 438 can be cooled in a separate circuit in mainheat exchanger 122 prior to introduction to the reflux drum 118. Ifstream 130 contains little methane, which can be rejected in the scrubcolumn 114 or in an additional demethanizer column in the fractionationsystem, then stream 330 can be completely condensed and there is no needfor the phase separator 430, there is no stream 436, and pump 434 canalso be eliminated. Further, stream 438 could be fed directly the scrubcolumn 114, for example to the second stage below the top of the column.

FIG. 5 shows a modification of the embodiment of FIG. 4, in which theseparated vapor portion 436 is compressed to the pressure of the scrubcolumn 114 in compressor 530, cooled and condensed in heat exchanger 532and the resultant stream 536 combined with the liquid portion 438 toform stream 538. Heat exchanger 532 could be a series of heatexchangers, the first one using cooling water, the other(s) usingvaporizing propane. Stream 438 can be warmed up to close-to ambienttemperature prior to compression in an additional heat exchanger, andcooled back down following the compression in the aftercooler and thesame additional heat exchanger for additional thermodynamic efficiency.Stream 536 may be a dense supercritical fluid.

FIG. 6 shows another modification of the embodiment of FIG. 4, in whichthe first overhead vapor 116 is partially condensed by cooling in heatexchanger 612, with, for example, either or both of the reflux stream136 and the absorber liquid 140 but preferably by vaporizing propane.The resulting first two-phase stream is separated in phase separator 618into a “second” overhead vapor stream 616, and a methane-rich liquidstream 619. The liquid stream 619 is returned to scrub column 114 asreflux. Stream 616, now at a temperature matching the temperatures atthe bottom of the main heat exchanger 122, is mixed with stream 438,cooled in main heat exchanger 122, and fed to the reflux drum 628 as atwo-phase stream 626. The overhead vapor stream 620 from the reflux drum628 is liquefied in the main heat exchanger 122 and recovered asliquefied natural gas product 124. The liquid stream 629 from the refluxdrum 628, optionally reheated in the main heat exchanger 122, isreturned to the scrub column 114 at the same or different location thanreflux stream 619.

Phase separator 618 and/or reflux drum 628 can be replaced withabsorption columns having two-phase feed at the bottom and refluxesprovided by cooled streams 136 and/or 140 at the top.

Individual features described in connection with any of the illustratedembodiments, or combinations of those features, can be incorporated asappropriate in any of the other illustrated embodiments. For example,the optional reheating of the reflux stream 629 in the main heatexchanger 122 described in connection with FIG. 6 can be applied to anyof the embodiments of FIGS. 1 to 5. Additionally or alternatively, theprovision of reflux to the scrub column 114 by a liquid fraction 619derived from the first vapor overhead 116 of the embodiment of FIG. 6also can be applied to any of the embodiments of FIGS. 1 to 5.

EXAMPLE

Using the embodiment of FIG. 3, 97,904 lbmol/h (44,408.5 kgmol/h) of apre-purified natural gas stream 110 at 950 psia (6.5 MPa) is cooled inheat exchanger 112 by three stages of propane cooling to −32.3° F.(−35.7° C.) and fed to the scrub column 114. This feed stream 110contains 0.6% nitrogen, 84.8% methane, 7.3% ethane, 4.4% propane, 0.7%isobutane, 1.5% butane, 0.3% isopentane, 0.2% pentane, and 0.2% hexanes.The column 114 operates at 840 psia (5.8 MPa) and has an intermediatereboiler heated by 40% of the stream 110 bypassing the first two stagesof propane cooling and a bottom reboiler 128 at about 130° F. (55° C.).Column overhead 116 is cooled from −62.3° F. (−52.4° C.) to −77.5° F.(−60.8° C.) in the warm bundle of the main heat exchanger 122 andintroduced into the reflux drum 118 as a two-phase stream containingabout 15% of liquid. Scrub column bottoms stream 126 is sent to thefractionation systems 128, consisting of a series of distillationcolumns comprising deethanizer, depropanizer, and debutanizer. 96% ofpropane present in the feed stream 110 is recovered as depropanizeroverhead. Nearly all of butane and isobutane is recovered as debutanizeroverhead. Deethanizer overhead containing about 39% of methane, 61% ofethane, and only 0.05% of propane at a flow rate of 6,105 lbmol/h (2,769kgmol/h) and pressure of 420 psia (2.9 MPa) is mixed with stream 140which constitutes 39% of the debutanizer bottoms liquid; the rest beingrecovered as C₅₊ product. Low propane content is important for highpropane recovery. Stream 140 is a liquid at 17 psia (117 kPa) and a flowrate of 406 lbmol/h (184 kgmol/h), and contains about 51% isopentane,36% pentane, 12% hexanes and less than 1% of lighter components. It ispumped, by a pump not shown in FIG. 3, to 420 psia (2.9 MPa) prior tomixing with ethane-enriched stream 130. The combined stream 330 iscooled in heat exchanger 332 by propane to −32.3° F. (−35.7° C.) andcompletely condensed by further cooling to −77.5° F. (−60.8° C.) in thewarm bundle of the main heat exchanger 122. The condensed stream ispumped to the scrub column pressure in pump 334 and introduced to thereflux drum 118. Liquid reflux 119 is returned to the top of the scrubcolumn 114 at −74.2° F. (−59.0° C.); there is a heat effect of pumpingand mixing in the phase separator. Stream 120, which contains 91.3%methane, 7.8% ethane, 0.7% nitrogen, 0.2% propane, and only traceamounts of heavier hydrocarbons, is at −74.2° F. (−59.0° C.) and has aflow rate of 83,571 lbmol/h (37,907 kgmol/h). It is cooled down to−161.6° F. (−107.6° C.) in the middle and cold bundles of the main heatexchanger 122 and then let down to the storage pressure of 15.3 psia(105.5 kPa) as liquid stream 124. The main heat exchanger 122 is cooled,as described with reference to FIG. 1, by mixed refrigerant comprisingnitrogen, methane, ethane, and propane.

It will be appreciated that the invention is not restricted to thedetails described above with reference to the preferred embodiments butthat numerous modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the followingclaims.

1. In a process for recovery of components heavier than methane from afeed of methane in admixture with ethane and heavier hydrocarbon(s) by aprocess comprising: introducing the feed into a scrub column at a firstlocation; withdrawing from the scrub column a first overhead vaporstream depleted in components heavier than methane and a bottoms streamenriched in components heavier than methane; cooling and partiallycondensing the first overhead vapor stream to form a first two-phasestream; separating the first two-phase stream to provide a secondoverhead vapor stream and a methane-rich first reflux stream;introducing the methane-rich first reflux stream at a second location inthe scrub column above the first location; and separating the bottomsstream into an ethane-enriched stream and one or more streams enrichedin components heavier than ethane; the improvement consisting in that anethane-enriched second reflux stream is derived from the ethane-enrichedstream and introduced into the scrub column at a location selected fromthe second location and a third location above the first location. 2.The process of claim 1, wherein the ethane-enriched stream is condensedat temperature below that of the feed to the scrub column and thecondensed stream pumped prior to introduction into the scrub column asthe ethane-enriched second reflux stream.
 3. The process of claim 2,wherein said temperature is below −32° F.
 4. The process of claim 1,wherein the feed is a cooled natural gas feed.
 5. The process of claim1, wherein the ethane-enriched second reflux stream is fed to the scrubcolumn separately from the methane-rich first reflux stream.
 6. Theprocess of claim 1, wherein the ethane-enriched second reflux stream ismixed with the methane-rich first reflux stream before introducing intothe scrub column.
 7. The process of claim 6, wherein the ethane-enrichedsecond reflux stream is combined with the first two-phase stream at alocation selected from upstream of and in a reflux drum.
 8. The processof claim 6, wherein the first two-phase stream is fed to the bottom ofan absorption column to which the ethane-enriched second reflux streamis fed as reflux.
 9. The process of claim 6, wherein the ethane-enrichedstream is fully condensed before mixing with the methane-rich firstreflux stream.
 10. The process of claim 1, wherein the ethane-enrichedstream is the vapor overhead of a deethanizer.
 11. The process of claim1, wherein an absorber liquid derived from the one or more streamsenriched in components heavier than ethane is introduced into the scrubcolumn at a location selected from the second location, the thirdlocation and a fourth location above the first location.
 12. The processof claim 11, wherein the absorber liquid (140) comprises pentane andisopentane.
 13. The process of claim 11, wherein the absorber liquidcomprises C₄ hydrocarbon(s).
 14. The process of claim 11, wherein theabsorber liquid comprises C₅₊ hydrocarbon(s).
 15. The process of claim11, wherein the absorber liquid is fed to the scrub column separatelyfrom either the ethane-enriched second reflux stream or the methane-richfirst reflux stream.
 16. The process of claim 11, wherein the absorberliquid is combined with at least one of the methane-rich first refluxstream and the ethane-enriched second reflux stream before introducinginto the scrub column.
 17. The process of claim 16, wherein the absorberliquid is combined with at least one of the first two-phase stream andthe ethane-enriched second reflux stream at a location selected fromupstream of and in a reflux drum.
 18. The process of claim 16, whereinthe first two-phase stream is fed to the bottom of an absorption columnto which at least one of the absorber liquid and the ethane-enrichedsecond reflux stream is fed as reflux.
 19. The process of claim 16,wherein the absorber liquid is combined with the first overhead vaporstream before partial condensation of said stream to form the firsttwo-phase stream.
 20. The process of claim 11, wherein the absorberliquid is combined with gaseous ethane-enriched stream beforecondensation of said stream to provide the second reflux stream.
 21. Theprocess of claim 20, wherein the combined absorber liquid and gaseousethane-enriched stream is phase separated and the liquid fraction is fedto the scrub column above the first location.
 22. The process of claim21, wherein the vapor fraction is condensed and fed to the scrub columnabove the first location.
 23. The process of claim 21, wherein theliquid fraction is combined with the first overhead stream beforepartial condensation of that overhead stream to provide the firsttwo-phase stream.
 24. The process of claim 21, wherein the condensedvapor fraction is combined with the first overhead stream before partialcondensation of that overhead stream to provide the first two-phasestream.
 25. The process of claim 24, wherein both the condensed vaporfraction and the liquid fraction are combined with the first overheadstream before partial condensation of that overhead stream to providethe first two-phase stream.
 26. The process of claim 1, wherein thefirst overhead stream is partially condensed in two stages and liquidfraction from each condensation is fed to the scrub column as reflux.27. The process of claim 1, wherein the methane-rich first reflux streamconstitutes at least about 80% of the total reflux (i.e. liquid fed tothe scrub column above the first location).
 28. The process of claim 1,wherein the second reflux stream is less than about 20% of the totalreflux (i.e. liquid fed to the scrub column above the first location).29. The process of claim 11, wherein the absorber liquid is less thanabout 10% of the total reflux (i.e. liquid fed to the scrub column abovethe first location).
 30. The process of claim 1, wherein the secondreflux stream contains less than about 0.05% of propane.
 31. The processof claim 1, wherein methane is removed from the ethane-enriched streamwhereby the second reflux stream consists essentially of ethane.
 32. Theprocess of claim 1, wherein more than 90% of propane is recovered fromthe bottoms stream as product.
 33. The process of claim 1, wherein morethan 90% of butane is recovered from the bottoms stream as product. 34.The process of claim 1, wherein second overhead vapor is liquefied toprovide liquefied natural gas (LNG) product.
 35. An apparatus forrecovery of components heavier than methane from a feed of methane inadmixture with ethane and heavier hydrocarbon(s) by a process of claim1, said apparatus comprising: a scrub column; conduit means forintroducing the feed into the scrub column at a first location; conduitmeans for withdrawing from the scrub column a first overhead vaporstream depleted in components heavier than methane and a bottoms streamenriched in components heavier than methane; heat exchanger means forcooling and partially condensing the first overhead vapor stream to forma first two-phase stream; separation means for separating the firsttwo-phase stream to provide a second overhead vapor stream and amethane-rich first reflux stream; conduit means for introducing themethane-rich first reflux stream at a second location in the scrubcolumn above the first location; separation means for separating thebottoms stream into an ethane-enriched stream and one or more streamsenriched in components heavier than ethane; and conduit means forintroducing into the scrub column, at a location selected from thesecond location and a third location above the first location, anethane-enriched second reflux stream derived from the ethane-enrichedstream.
 36. The apparatus of claim 35, comprising heat exchange meansfor condensing the ethane-enriched stream at temperature below that ofthe feed to the scrub column and pumping means for pumping the condensedstream prior to introduction into the scrub column as theethane-enriched second reflux stream.
 37. The apparatus of claim 35,comprising a reflux drum from which a mixture of the ethane-enrichedsecond reflux stream and the methane-rich first reflux stream is fed tothe scrub column.
 38. The apparatus of claim 35, comprising anabsorption column which receives the first two-phase stream as a bottomfeed and the ethane-enriched second reflux stream as reflux and fromwhich the bottoms liquid is fed to the scrub column.
 39. The apparatusof claim 35, comprising conduit means for introducing into the scrubcolumn, at a location selected from the second location, the thirdlocation and a fourth location above the first location, an absorberliquid enriched in component(s) heavier than ethane and provided by theseparation means for separating the bottoms stream of the scrub column.40. The apparatus of claim 39, comprising a reflux drum from which amixture of the absorber liquid and at least one of the methane-richfirst reflux stream and the ethane-enriched second reflux stream is fedto the scrub column.
 41. The apparatus of claim 39, comprising anabsorption column which receives the first two-phase stream as a bottomfeed and at least one of the absorber liquid and the ethane-enrichedsecond reflux stream as reflux and from which the bottoms liquid is fedto the scrub column.
 42. The apparatus of claim 39, comprising means forcombining the absorber liquid with the first overhead vapor streambefore partial condensation of said stream to form the first two-phasestream.
 43. The apparatus of claim 39, comprising means for combiningthe absorber liquid with gaseous ethane-enriched stream beforecondensation of said stream to provide the second reflux stream.
 44. Theapparatus of claim 43, comprising separator means for phase separatingthe combined absorber liquid and gaseous ethane-enriched stream andconduit means for feeding the liquid fraction to the scrub column abovethe first location.
 45. The apparatus of claim 44, comprising heatexchange means for condensing the vapor fraction and conduit means forfeeding the condensed vapor to the scrub column above the firstlocation.
 46. The apparatus of claim 44, comprising means for combiningthe liquid fraction with the first overhead stream before partialcondensation of that overhead stream to provide the first two-phasestream.
 47. The apparatus of claim 45, comprising means for combiningthe condensed vapor fraction with the first overhead stream beforepartial condensation of that overhead stream to provide the firsttwo-phase stream.
 48. The apparatus of claim 47, comprising means forcombining both the condensed vapor fraction and the liquid fraction withthe first overhead stream before partial condensation of that overheadstream to provide the first two-phase stream.
 49. The apparatus of claim35, comprising means for partially condensing the first overhead streamin two stages and feeding the liquid fraction from each condensation tothe scrub column as reflux.